1. Field of the Invention
The present invention relates generally to a method and apparatus for fabricating a silica-based glass using an optical fiber preform. More particularly, the present invention relates to a method and apparatus for fabricating a transparent substrate tube or an over-jacketing tube by a sol-gel process.
2. Description of the Related Art
In general, a silica-based glass is transparent and chemically inert, and has a high level of thermal stability and strength, and a low thermal expansion coefficient. These valuable characteristics make the silica-based glass useful for an optical fiber preform.
An optical fiber includes a core layer having a predetermined refractive index therein, and a cladding layer having a lower refractive index different from the core such that incident light is totally reflected within the core layer. Normally, in order to fabricate an optical fiber, an optical fiber preform is prepared and thermally treated, then extended to form a strand of optical fiber. The extended optical fiber can be coated to form an optical fiber cable.
Alternatively, an over-cladding or an over-jacketing process can be implemented to obtain an optical fiber preform with a larger diameter that can produce longer optical fibers. Such an optical fiber preform is prepared by either the chemical deposition method or the sol-gel process. The chemical vapor method involves the fabrication of a solid silica-based glass through vapor reaction, but the productivity using this method is low. Also, the chemical vapor method requires operation at a high temperature for the fabrication process which in turn raises fabrication cost due to expensive equipment needed to perform at high temperature. The sol-gel process, however, is more economical and has some advantages of achieving high productivity because of its liquid process, and has capabilities to freely adjust the component formation and process at a relatively low temperature. Moreover, the sol-gel process uses a high purity material as the starting material, and this is very useful for fabricating a high purity silica glass.
The sol-gel process for fabricating the silica glass by using a high purity material is disclosed in U.S. Pat. No. 5,912,397 by Young-Min Baik, titled xe2x80x9cHigh-purity silica glass fabrication method using sol-gel processxe2x80x9d, and U.S. Pat. No. 5,919,280 by Jeong-Hyun Oh, titled xe2x80x9cMethod for fabrication silica glassxe2x80x9d.
In addition, an apparatus and method for sintering a silica-based glass in a sintering furnace are disclosed in detail in U.S. Pat. 5,423,898, titled xe2x80x9cMethod of lowering and raising an optical fiber preform in a sintering furnace.xe2x80x9d This patent discloses a sintering phase of the sol-gel process. Another apparatus for fabricating a transparent silica glass preform which is disclosed in the U.S. Pat. No. 5,423,898 by Masami Terashima, et al., includes a fixed furnace body, a furnace tube, and a porous silica glass body that moves longitudinally along the furnace tube. The furnace and the furnace tube are fixed and only the transparent silica-based glass is vertically movable. This type of configuration has some drawbacks. As the furnace and the furnace tube are permanently fixed, the silica glass has to move along the furnace tube and around the furnace, then the sintered glass is removed. Such configuration requires a sintering tower with the height that reaches almost 10 m, thus, it becomes very difficult to maintain the alignment of the equipment and the silica glass during the fabrication stage since the silica glass subject has to be moved. Also, manufacturing costs of the equipments tend to increase to implement this type of equipment. Furthermore, the height of a factory zenith is determined depending on the height of the equipment; hence, maintenance and usage costs are costly, which in turn raises the production cost.
It is, therefore, an object of the present invention to provide an apparatus and method for sintering an over-jacketing tube, in which the height of a sintering tower required in the sintering process is lowered compared to the conventional art system, so as to easily align of the equipment and the processing tube.
It is another object of the present invention to provide an apparatus and method for sintering an over-jacketing tube, in which the manufacturing cost of a sintering tower that is required in the sintering process is lowered compared to the conventional art system.
It is still another object of the present invention to provide an apparatus and method for sintering an over-jacketing tube, in which maintenance and operation costs of the sintering tower are less than the conventional art system.
To achieve the above objects, there is provided an apparatus and method for sintering an over-jacketing tube. The apparatus for sintering the over-jacketing tube includes a tube, a gel tube disposed within the tube, and a furnace for thermally treating the gel tube. The apparatus includes a processing tube composed of quartz; a gel tube assembly including a top rotation cap positioned at the top of the processing tube, for sealing the inside of the processing tube and rotating at a regular peripheral velocity, a ceramic pin elongated downward from an axis of the top rotation cap, and a gel tube suspended from a bottom of the ceramic pin and disposed on the same axle within the processing tube; and a movable furnace initially disposed on the bottom of the processing tube, and moving upward along the processing tube after the gel tube assembly is inserted into the processing tube so as to thermally process the gel tube.
Accordingly, the method for sintering the over-jacketing tube with a device of the type having a processing tube; a gel tube assembly having a top rotation cap and a prepared gel tube, from which the prepared gel tube is suspended, and disposed within the processing tube; and a furnace for thermally processing the gel tube, the method comprising the steps of: inserting the gel tube assembly into an axial bottom of the processing tube; injecting helium gas into the inside of the processing tube; rotating the gel tube at a regular peripheral velocity; moving the furnace along the processing tube in a vertically upward direction and therefore thermally processing the gel tube; and, stopping the injection of the helium gas as well as moving the furnace to its initial position at a high velocity after thermally processing the gel tube.